In recent years, an attempt to reduce the weight of automobiles or various machine parts has been made. The reduction in weight can be realized by the optimization design of the part's shape to ensure rigidity. In the case of hollow parts such as press-formed parts, the reduction in weight can be directly realized by reducing the plate thickness. However, in order to maintain the static fracture strength and the yield strength while reducing the plate thickness, it is necessary to use a high-strength material for the parts. For this purpose, an attempt to apply a steel sheet having a tensile strength of 590 MPa or more to a low-cost steel material having excellent strength properties has been made. Meanwhile, in order to highly strengthen the material, it is necessary to satisfy both of high strength and formability such as fracture limit during shape forming or burring formability. Furthermore, when the parts are applied to chassis parts, a steel sheet based on precipitation-strengthening by the addition of micro-alloy elements has been developed in order to ensure toughness of an arc-welded part and to suppress HAZ softening. In addition to this, various steel sheets have been developed (for example, see Patent Documents 1 to 5).
The above-described micro-alloy elements promote the precipitation of coherent precipitates of approximately several nanometers to several tens of nanometers in size at a temperature below the Ac1 temperature. In the process of manufacturing the hot-rolled steel sheet, the strength of the steel sheet can be significantly improved by such coherent precipitates, but there is a problem in that fine cracks are generated at a sheared edge and formability is deteriorated, as disclosed in Non-patent Document 1 for example. Furthermore, the deterioration in a sheared edge significantly deteriorates fatigue properties of the sheared edge. In Non-patent Document 1, this problem was solved by utilizing microstructure strengthening while using alloy constituents to which micro-alloy elements were added. However, when the microstructure strengthening is utilized, it is difficult to achieve a high yield strength required for the parts, and the suppression of the deterioration of the sheared edge of the precipitation-strengthened hot-rolled steel sheet remains an issue.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-161340    Patent Document 2: JP-A No. 2004-27249    Patent Document 3: JP-A No. 2005-314796    Patent Document 4: JP-A No. 2006-161112    Patent Document 5: JP-A No. 2012-1775    Non-patent Document 1: Kunishige et al., TETSU-TO-HAGANE, vol. 71, No. 9, pp. 1140-1146 (1985)